1. Field of the Invention
The invention relates to a comparison circuit for comparing two signals with each other and to ascertain whether these signals are equal to each other. This comparison circuit is particularly intended for use in a car radio receiver.
2. Description of the Prior Art
As is generally known a car receiver includes a tunable receiving circuit. By applying a predetermined tuning datum to this receiving circuit it is tuned to a predetermined radio channel through which a certain program is transmitted. Modern car radios often comprise a memory in which the tuning data for a number of radio channels are stored. Such a tuning datum can then be selected by means of a selection switch and applied to the receiving circuit.
Reference 1 discloses a car receiver in which the memory consists of a plurality of addressable memory fields each having a plurality of addressable memory locations. A tuning datum is stored in each memory location and a memory field contains more in particular the tuning data for, for example, all those radio channels through which the same program is transmitted. One of these memory fields can be selected by means of the selection switch. A search tuning circuit has been provided for selecting one of the tuning data stored in this selected memory field. This search tuning circuit has a search-mode and a rest-mode. If it is adjusted to the search-mode, the tuning datum which corresponds to the radio channel whose received field strength is strongest is selected from the tuning data stored in the relevant memory field. After the receiving circuit has been tuned to this radio channel the search tuning circuit is automatically adjusted to the rest-mode. The search tuning circuit is automatically adjusted to the search mode again when the selection switch is operated, but also when the received field strength of the radio channel to which the receiving circuit has been tuned decreases to below a predetermined value.
This prior art car radio receiver has the disadvantage that all tuning data must be previously fed manually into the different memory locations. The consequence thereof is, for example, that if the receiving circuit must be tuned manually to a radio channel the tuning datum of which has not been previously stored in one of the memory fields and the received field strength of this radio channel decreases to below a desired value that then this receiving circuit must be readjusted manually.
To obviate the two above-mentioned disadvantages, reference 2 proposed to tune the receiving circuit (for example manually) to a radio-channel through which a desired program is transmitted and to search thereafter, for example by means of an auxiliary receiving circuit to which a search tuning circuit is connected, for all those radio channels through which the same program is transmitted and to store the tuning data obtained therefrom in a memory field. In order to ascertain whether the desired program is also transmitted through that radio channel to which the auxiliary receiving circuit has been tuned by the search tuning circuit, the output signals of the two receiving circuits are compared with each other in a comparison circuit. This comparison circuit produces a first output signal when the two signals are equal to each other. If they are not equal, the comparison circuit produces a second output signal. In response to the first output signal the tuning datum which was used to tune the auxiliary receiving circuit to the relevant radio channel is stored in a memory location of a memory field.